I recently purchased a Garmin 489 and heard all about the external vs internal debate that is all the rage at my dock. I was told my Bimini would block the signal and a wet Bimini is worse. I heard that a my signal stength would be diminished and take more time to aquire a signal but I got such a deal on my unit $500.00 +tax Brand new in box that I decided to take the risk. Presently I have not installed it on the boat, However I hooked it up to a deep cell 12v battery in my living room and it was sleeting outside and I still recieved 8 satellites with strong signals and the unit states that the present accuracy is 19 feet.
me thinks it will work just fine under canvas compared to 1/2 plaster, 6" insulation,3/4 plywood and 3 layers of shingles.
Disclaimer: I could have a defective unit that doesn't know it's not suppose to work under those conditions. Now back to couch sailing with my GPS till it warms up outside.